Mocca Man is Impossible, Human Bio-Diversity is Guaranteed to Increase Exponentially
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In the absence of a generation-time effect, the observed decrease in hominoid substitution rates must be due to either a decrease in the per-cell-division mutation rate or a decrease in the germline cell division rate. The predictions laid out in figure 1 show that decreased rates of cell division would lead to lower values of α, which is contrary to the observed trends. Therefore, the data imply that there has been a decrease in the per-cell-division mutation rate (µc) in hominoids and that this rate is further decreasing in humans.
On the other hand, because the per-generation mutation rate (µg) is determined by the accumulation of mutations across many germline cell divisions, consideration of recent demographic shifts in human populations suggests that µg is actually increasing. In essence, the increased rate is simply a result of increases in the average human generation time, which is much longer now (25–30 years; Fenner 2005) than it was in archaic humans (18–19 years; Hemmer 2007). Even within the last 40 years, data from developed countries show an increasing average generation time for both females (Bongaarts 2001) and males (Svensson et al. 2011). Taken together with the fact that mutation rates increase with paternal age, these increases in generation time result in higher per-generation mutation rates.
Experimental manipulation of the age at reproduction in mutation-accumulation experiments has shown that increased generation times result in increased µg (Latta et al. 2013). In particular, increased generation times lead to increased per-generation deleterious mutation rates and increased variance in fitness among individuals. If similar increases in the variance in fitness among humans occur as a result of increases in µg, such changes may have important consequences for understanding the ongoing evolution of human health (cf. Nesse and Williams 1994; Stearns and Koella 2007).
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